1. Field of the Invention
This invention generally relates to FM (frequency modulation) demodulators. More particularly, the present invention relates to an FM demodulator which determines the logic state of current input-data by performing level comparison of the current input-data and the preceding input-data and which reproduces data in accordance with the determination, thereby performing FM demodulation of a received Bluetooth radio signal.
2. Description of the Related Art
Typically, many radio signal receivers that reproduce FM signals incorporate FM demodulators of the type that perform FM demodulation of received signals by utilizing a resonating circuit having an S-shaped characteristic curve. In such an FM demodulator, when a received signal is supplied to the resonating circuit having the S-shaped characteristic curve, the resonating circuit outputs an FM demodulated signal whose level is shifted from a reference level toward a positive- or negative-level in accordance with the direction and amount of frequency displacement from the FM signal carrier frequency. When the FM demodulated signal contains binary data, the positive- or negative-level FM demodulated signal output from the resonating circuit is supplied to a level determination unit. At this point, the level determination unit performs level comparison of the positive- or negative-level FM demodulated signal with the average integrated level of previous FM demodulated signals. When the level comparison result indicates that the level of the current FM demodulated signal is greater than the average integrated level of the previous FM demodulated signals, the level determination unit generates a demodulated output indicating a high (H) level as data. On the other hand, when the current demodulated signal level, is smaller than the average integrated level of the previous FM demodulated signals, the level determination unit generates a demodulated output indicating a low (L) level as data.
Recently, as a new method for transmitting radio signals between apparatuses such as portable telephones and personal computers without the need for wire connections, a radio-signal transmission system employing Bluetooth technology has been brought to attention. The Bluetooth radio-signal transmission system employs the 2.4-GHz band, which has been set aside, for its frequency band. While the effective distance of transmitted radio signals is limited to quite a short distance of about 10 meters, the Bluetooth system is robust and simple and also has the feature of being able to transfer data with low power consumption and at low cost.
In this case, the Bluetooth radio-signal transmission system employs an FM system as the method for modulating information signals. Since the effective transmission distance is about 10 meters, which is short, the system is effectively used to wirelessly link apparatuses in a single room in an office or the like. Possible applications include a connection between the main part of a personal computer and a peripheral device such as a mouse or keyboard, a connection between a portable telephone and a notebook computer, and a connection between a stereo set and headphones. Other potential applications include a connection between a portable telephone and a vending machine, and a connection between a portable telephone and a parking meter in a parking lot, provided that they are located in close proximity to each other.
Since the Bluetooth radio-signal transmission system was developed on the premise that the system is incorporated in existing apparatuses in order to allow the apparatuses to communicate with each other, the configuration of the system must be low cost. As a result, in the specification of the Bluetooth radio-signal transmission system, the frequency tolerance with respect to the carrier wave or modulation wave is relatively large. Thus, when the FM demodulator of a receiver is configured with a resonating circuit having an S-shaped characteristic curve as described above, since the specification is relatively loose, the FM demodulator cannot form a demodulated signal (demodulated data) with high determination accuracy. For example, when an FM signal is supplied to the resonating circuit to provide a positive- or negative-level demodulated output, level comparison of the demodulated output level and the absolute level is performed in a level determination unit, and based on the level comparison, level determination is made as to whether the output data is at the high (H) level or the low (L) level, there is difference in some cases between the determined level and the level of the current data.
In order to address such a problem, the Bluetooth radio-signal transmission system is designed to be able to determine whether the demodulated data is at the high (H) or low (L) level by identifying the relative levels of the demodulated data and data adjacent to the demodulated data. With respect to data transmitted in the radio signal, the numbers of the high (H) levels and low (L) levels are adapted to become generally uniform by Gaussian Frequency Shift Keying (GFSK).
The present invention has been made in view of the foregoing technical background, and an object thereof is to provide an FM demodulator which can determine whether the demodulated data is at the high (H) or low (L) level with a simplified configuration and which can provide demodulated data with high determination accuracy.
To achieve the object, there is provided an FM demodulator for reproducing data by performing FM demodulation of a received RE signal in a Bluetooth transmission system. The FM demodulator includes a frequency discrimination unit and a level determination unit. The FM demodulator frequency-discriminates the RF signal, performs digital conversion of the discriminated RF signal, and outputs converted data. The level determination unit performs level comparison of current input-data, supplied from the frequency discrimination unit in the latest clock period, with last input-data, supplied from the frequency discrimination unit in the immediately preceding clock period, determines a logic state of the current input-data based on the result of the level comparison, and reproduces data in accordance with the determined logic state.
According to the present invention, the current input-data supplied from the frequency discrimination unit for each new clock period is compared with the last input-data supplied from the frequency discrimination unit in the immediately preceding clock period to determine the relative level of the current input-data with respect to the last input-data. The logic state of the current input-data is determined based on the determination result, thereby providing an advantage that demodulated binary data can be reproduced with high determination accuracy.
Preferably, the level comparison of the current input-data with the last input-data is performed by a current-data register for retaining the current input-data, a one-clock-before-data register for retaining the last input-data, and a comparator for comparing the output states of the two registers.
With this arrangement, the level comparison of the current input-data and the last input-data uses only two registers and one comparator, thereby providing an advantage that the level comparison can be performed with a simplified configuration.
Preferably, the determination of the logic state of the current input-data is performed by a current-level flip-flop and an output-control flip-flop which have different output states in response to the result of the level comparison of the current input-data and the last input-data, and by a controllable amplifier having an output logic value that is controlled in accordance with the output states of the two flip-flops.
With this arrangement, the determination of the logic state of the current input-data uses only two flip-flops and one controllable amplifier, thereby providing an advantage that the logic state can be determined with a simple configuration.